This invention relates to a process of purifying exhaust air which is laden with organic pollutants, particularly exhaust air from paint-drying operations, comprising an adsorption on particulate activated carbon, which is contained in a fixed bed and from which at least part of the adsorbate can be desorbed.
It is known to feed exhaust air of the above-mentioned kind of a furnace, in which the pollutants are burnt without a residue at temperatures of 750.degree. to 850.degree. C. The resulting hot exhaust gases may then be used, e.g., to heat the paint dryers and to preheat the exhaust air which is to be purified.
Another known process comprises an adsorption of the pollutants on activated carbon. Activated carbon has been used for a long time particularly for the removal of organic solvents having boiling points up to about 140.degree. C. When the activated carbon has been exhausted, i.e., when it is laden to saturation, it is regenerated by a treatment with hydrogen to remove the adsorbate. It has not been possible before to use this regeneration in connection with the purification of exhaust air from paint-drying operations.
Exhaust air from paint-drying operations contains various substances from different paint coats or from additional protective layers, such as an underbody coating. For this reason the pollutants contained in the exhaust air include solvents and plasticizers. Most of these substances have boiling points in the range of about 150.degree. to 220.degree. C.
But even under a reduced pressure of 25 millibars, the plasticizers which are used do not boil at temperatures below 255.degree. to 265.degree. C. During the baking of the coats, said substances enter the exhaust air from the dryers and are subsequently adsorbed on the activated carbon. Besides, cracked and polymerized products are formed at an uncontrolled rate and are also adsorbed on the activated carbon.
For these reasons it can be understood that it was previously believed that very high temperatures were required to regenerate activated carbon laden with such substances. To prevent a burning of the activated carbon, care had to be taken to ensure that the desorbing gas is virtually free from oxygen. In the known process, the activated carbon was regenerated by a treatment with hot inert gases, which were produced, e.g. by a combustion of propane or fuel oil and were at temperatures of about 500.degree. C. The laden desorbing gases were fed to a thermal combustion plant. A process based on this principle has been described in the periodical "Oberflache+JOT" (1978) on pages 775 to 777.
It has been found that in that known process the bulk density of the carbon increases progressively in spite of the high desorption temperatures because the quantity of non-desorbable matter increases steadily. As a result, the time until the activated carbon is fully laden decreases from cycle to cycle. The increase of the bulk density of the carbon as a result of the deposition of polymerized products on the carbon can be compensated in that the carbon is re-activated at temperatures of about 750.degree. C., although this is accompanied by a burning of carbon. For such re-activation, it is also necessary either to discharge the spent activated carbon from the adsorber and to re-activate the carbon externally in a special furnace, or to design the adsorbers also as re-activating furnaces. In the first case, a substantial proportion of fines will be braded from the carbon in transit; that proportion may amount to some percent per re-activation. The second alternative involves a high expenditure of expensive refractories. In both cases, additional losses are due to the inevitable burning of activated carbon.